Problem: The scale on a map is 2cm : 1km. If the distance between two cities is 3km, how far apart in cm are the two cities on the map?
The scale means that every 2cm on the map corresponds to an actual distance of 1km. An actual distance of 3km is the same as 3 $\cdot$ 1km. The distance between the two cities on the map is 3 $\cdot$ 2cm, or 6cm.